Methods of compounding silica into tires have become well known in recent years in response to increasingly stringent demands for higher tire performance. However, silica has a tendency to aggregate due to the formation of hydrogen bonds caused by silanol groups present on the particle surface thereof, and is typically difficult to increase the dispersibility thereof.
Therefore, a technique of introducing a functional group which interacts with silica into the polymer constituting the matrix of a rubber composition has been proposed. However, such a polymer exhibits strong silica/polymer interactions and polymer/polymer interactions, which is problematic in that the viscosity of the unvulcanized rubber becomes high, thereby diminishing processability.
Therefore, the viscosity can be reduced by increasing the compounded amount of a softening agent such as a process oil. However, in this case, there are problems in that the hardness decreases so that the steering stability is lost, while the tan δ (60° C.) is diminished, which makes it impossible to achieve the desired low fuel consumption.
There have also been attempts to increase silica dispersibility by using a surfactant as a dispersant. For example, a technique of using diethylene glycol is disclosed in Japanese Unexamined Patent Application Publication No. H8-302077A, and a technique of using a fatty acid and trimethylolpropane is disclosed in Japanese Unexamined Patent Application Publication No. 2006-52407A. However, in each of the conventional technologies, sufficient silica dispersibility has not been achieved, and it has not been possible to simultaneously enhance hardness and low fuel consumption without diminishing processability and hardness.